Thermo-programmed desorption is a widely used analysis technique in which the gas molecules absorbed by a solid surface are extracted by thermal heating. Since its beginnings in the 1940's it has only been applied to the desorption of gases. In 1990 IUPAC (International Union of Pure and Applied Chemistry) described it as an experimental technique for characterizing surfaces (Pure and Applied Chemistry vol 62, No. 12 pp 2297-2322, 1990) and they too make reference only to the desorption of gases. The present invention can be considered as the first piece of equipment described for carrying out PTD analysis in solution, opening up the possibility of extending the technique to a great many areas of research.
In a typical thermo-programmed desorption experiment a small amount of solid containing an absorbed gas is introduced into a reactor arranged inside an oven. The reactor is heated, generally following a linear increase in temperature with time. As the temperature rises the absorbed gas desorbs. An inert gas, generally helium, flows through the reactor and carries the desorbed gas molecules towards a detector. Alternatively the molecules are drawn by a vacuum.
A small thermocouple inserted inside the reactor measures the temperature while the detector in contact with the current of carrier gas analyzes the concentration of the gas desorbed. The response of the detector is proportional to the rate of desorption. This rate increases with temperature, reaches a maximum value and returns to zero when the surface is completely empty.
The desorption spectrum (thermogram) is a recording of the concentration of the gas desorbed as a function of temperature. Normally the spectrum can exhibit more than one maximum (peak).
The number, shape and position of the peaks, as well as the area contained within the thermogram, hold a great deal of information about the gas, the surface and the interaction between the two.
The thermo-desorption technique has its origins in the 1930's when URBACH, in is experiments on luminescence, observed the escape velocity of electrons from a continuously heated material. However, the application of this idea to the study of the interaction between gases and solids took place somewhat later.
The first work which refers to desorption itself was carried out by APKER and is described in his studies published in 1948 about the existing methods of measuring low pressures. These studies describe the difficulty in using ionization manometers as a result of the surface contamination of the filament by the absorption of gases, but show, nevertheless, that when subjected to abrupt heating, flash, there was a sudden increase in pressure due to the desorption of said gases.
In 1953 in the Bell Telephone laboratories (Murray, N.J.) HAGSTRUM designed and built several pieces of apparatus for studying the extraction of electrons from metal surfaces by bombardment with positive ions. These experiments show the importance of working with surfaces which are atomically clean. One indication of this contamination was the increase in pressure which took place when said surfaces were heated quickly to high temperature 1750 K.(Mo) or 2200 K. (W). Furthermore he observed that this increase was not uniform with temperature but could have maximum values.
In the same laboratories it was shown that the rate of gas desorption is dependant on temperature. The experiment was carried out in a vacuum system where an auxiliary filament of W or Mo was heated using a continuous current. An electronic circuit was designed to display the increase in pressure against the temperature on an oscilloscope screen. In this way the first desorption thermogram was obtained, i.e. the first representation of a variable related to the amount desorbed against temperature.
From this date on flash desorption began to develop widely, the heating process varying between 10 and 1200 K./s. In general, the equipment and procedures used were very similar. The solid under investigation was immersed in a gas connected to a vacuum system in which was located a device able to produce rapid heating. The amount of gas desorbed from the sample during the heating process could be determined by the increase in pressure inside the system, generally by means of an ionization manometer. By passing a current of gas to be absorbed by the surface after the flash, the equipment was once again ready for carrying out another desorption experiment.
Many studios have been carried out using the flash desorption equipment of the type described above. The first experiments concentrated on studying the absorption states of diatomic gases by W, while at the same time the theory required for the quantitative analysis of the experiments was developed. Later on said experiments dealt with the phenomena of interaction and interchange between gases absorbed by a surface. By 1963, the flash desorption technique had been more or less perfected. Among the many studies examined, it is worth mentioning the one carried out by AMENOMIYA and CVETANOVIC regarding the interaction of ethylene with a surface of aluminium oxide. The apparatus was fitted with a controller, which enabled various linear rates of heating to be set, and a thermal conductivity thermistor for detecting the ethylene desorbed and carried along by a current of helium. Since the surface was non-metallic the rates of heating were much lower, between 0.5 and 40 K./min. The recorded desorption rate increased with temperature and later decreased as the absorbed gas was used up, tracing a peak. At the same time the temperature of the system was picked up by another recorder connected to the thermocouple.
From the experimental point of view the need to determine partial pressures in the gas phase of a system stimulated the use of various types of mass spectrometer, this kind of detector finding a clear application in the study of isotopic surface interchange reactions as well as for the study of the decomposition of substances absorbed by surfaces.
Another contribution in the field of thermo-desorption which is worthy of mention is that of CZANDERNA which deals with following the desorption process by direct weight using a microbalance. In this way it is possible to obtain a more direct measurement and work at high pressures. The studies of FARNETH are along the same lines and deal with the mechanism of oxidation of alcohols on MoO.sub.3 where the desorption process was studied simultaneously by means of a balance and a mass spectrometer.
More recently the technique of programmed temperature desorption found an important application in the study of catalytic process. It was of course necessary to modify somewhat the previously described equipment as well as the process, due principally to the porous structure of the catalytic materials as opposed to the relatively uniform surface of the metallic materials which had previously been used.
Of the first work carried out it is worth mentioning that of CVETANOVIC and AMENOMIVA.
Their first study involves the modifications which have to be made to the flash desorption equipment. An oven was used to increase the temperature of the catalyst and an inert gas, helium, was used to carry the sample desorbed which was then analyzed by a chromatograph. The rates of heating were much lower, between 10 and 30 K./min such that the system remained close to a position of equilibrium between absorption and desorption.
Once the equipment had been modified the authors in their subsequent work moved on to the study of different catalytic systems: butene/aluminium, propylene/aluminium, ethylene/aluminium.
Later on slight modifications were introduced, relating principally to the means of detecting the species desorbed. This is the case for the equipment designed and perfected by MENON which uses a chromatograph as a detector in the study of n-pentane on Pt-Al.sub.2 O.sub.3, the same as ANDERSON in his work on the desorption of hydrogen from the catalysts Pt and Au. Another means of detection is described in the work by TOPSOE which was to study the desorption of ammonium and pyridine from zeolites. In both cases infrared spectroscopy was used as the identification technique. The rate of heating varied between 5 and 40 K./min.
A more sophisticated modification was made by the investigators LATZEL and KAES who built a piece of apparatus in which the sample desorbed was drawn along by vacuum and which could function automatically. Both the oven and the type of heating were regulated by a computer which also controlled the mass spectrometer used as a detector and at the same time collected and stored all the data such as m/e, intensities, time, temperature, etc.
By the beginning of the 1980's the experimental equipment had already been more or less perfected. There have therefore been very few modifications since that time and work on thermo-desorption basically describes the results obtained or the theoretical considerations concerning the technique. A typical diagram of the apparatus from this period is included in the study by FALCONER.
Before concluding, there are two further issues worth mentioning: one is the changes in the rate of heating, and the other is the increase in complexity of the surfaces to be studied.
At first, the heating processes involved in flash desorption were very abrupt and poorly controlled, varying between 10 and 1200 K./s. As the equipment was perfected, this rate was reduced accordingly. For example, RIGBY worked with rates of heating between 5 and 32 K./s and years later AMENOMIYA and CVETANOVIC managed to work with rates of between 0.5 and 40 K./min. This reduction lead to the modification of the temperature detection system, the sensitivity of the thermocouples being insufficient, and enabled the problem of temperature gradients set up in the absorbent to be solved.
It is also worth mentioning the introduction of non-linear heating programmes such as those in which temperature and time vary reciprocally (hyperbolic heating). Hyperbolic heating implies greater complexity from the experimental point of view, but at the same time can improve the resolution of the thermogram and simplify the processing of the equations.
In 1962 REDHEAD published his work concerning the theoretical aspects of determining the activation energy, using the rates and orders of reaction for both types of heating, linear and hyperbolic, to make a comparative study.
In more recent years, studies on the thermo-programmed desorption of ammonium absorbed by zeolites using hyperbolic heating have shown that the kinetic parameters obtained with this procedure are more accurate than those obtained with linear heating and avoid the fairly frequent drawback of the sometimes observed dependence of these parameters on the rate of heating.
With regard to the surfaces studied the technique has undergone a long evolution. Initially, as has already been mentioned, the aim was to eliminate the absorbed contaminants absorbed by the filaments of ionization manometers. However, within a short time interest was centered instead on the absorption of these gases by metal surfaces and there are a great many studies concerning the absorption of nitrogen, hydrogen and carbon monoxide by metals, in most cases W. The reason for this continued interest is the direct relation to catalysis.
Later on the technique was applied to the study of more complex surface phenomena such as the desorption of the decomposed species from the surface, or those formed by catalytic effects. This is the case of the desorption of some organic compounds (ethane, methane, benzene) absorbed by metal surfaces such as W, Tr or Pt.
Once the necessary modifications in the equipment had been achieved and the application of the technique had been extended to the study of catalytic effects, work broadened further to cover porous catalysts. It is therefore worth mentioning the study of the absorption/desorption of hydrocarbons and alcohols by catalysts such as aluminium, carbon, silica gel, magnesium oxide, etc.
In recent years programmed desorption has also been used to characterize supported metals. The technique is now widely used for both porous and metal catalysts, or for metal oxide catalysts, and constitutes a valuable tool for the study of absorption/desorption surface phenomena as well as catalysis. Finally and as has already been shown, the application of this technique has only been carried out in the gas phase but never in the condensed phase. This could be due perhaps to the difficulty in reaching the required temperatures for desorption under these conditions, or to the scarcity of work and little development in the research into desorption in solution. The equipment designed therefore widens the field of thermo-programmed desorption.